1. Field of the Invention
The present application deals with the field of microwave electronics, more particularly to baluns for millimeter wave applications.
2. Art Background
Millimeter wave baluns are used in a number of applications, such as antenna feeds, balanced mixer feeds, couplers, transitions from one guiding structure to another, and the like.
Because of the extremely broad use of microstrip transmission line (an unbalanced line), a number of microstrip baluns, converting from an unbalanced to a balanced configuration, have been introduced. Basraoui achieved an octave bandwidth at 2 GHz using a log-periodic structure of half-wave resonators, exchanging circuit size for bandwidth. Qian implemented a simpler, more compact structure using a power divider and simple phase shifter, but achieved slightly less than an octave bandwidth at 7GHz. Rogers achieved nearly two octaves of bandwidth, 6 to 18 GHz using a power divider and Lange couplers as phase shifters, but again at the cost of significant circuit real-estate.
Previous solutions depend on frequency limiting structures to provide power division and phase shifting. What is needed is a balun covering the 20-50 GHz millimeter-wave frequencies which is compact in size.
Energy coupled to an input microstrip line on one side of a substrate is coupled equally into two slotline arms on the opposite side of the substrate. Each of the slotline arms is then coupled to an output microstrip line on the same side of the substrate as the input microstrip line. By changing the physical configuration of the transition from slotline to microstrip line between the two slotline arms, a phase shift of 180 degrees is imposed between the two output microstrip lines. A phase equalizer section is used in one of the output striplines to ensure that the physical lengths of both output microstrip lines are the same so that no additional phase shift is introduced.